Semiconductor device and method of forming the same

ABSTRACT

A semiconductor device may be provided, including a substrate which includes a first semiconductor layer having a well region arranged within the first semiconductor layer, a buried insulator layer arranged over the first semiconductor layer, and a second semiconductor layer arranged over the buried insulator layer. The semiconductor device may include a capacitive structure including: the well region, at least one contact to the well region, at least a portion of the buried insulator layer over the well region, at least a portion of the second semiconductor layer, a source region and a drain region arranged over the second semiconductor layer, a gate dielectric layer arranged over the second semiconductor layer and arranged laterally between the source region and the drain region, and a gate layer arranged over the gate dielectric layer. The well region, the source region, and the drain region may have the same conductivity type.

TECHNICAL FIELD

The present disclosure relates generally to a semiconductor device and amethod of forming a semiconductor device. In particular, the presentdisclosure relates to a capacitor device and a method of forming acapacitor device.

BACKGROUND

Capacitors are widely used in complementary-metal-oxide-semiconductor(CMOS) integrated circuits. A typical type of capacitor is aMetal-Oxide-Semiconductor (MOS) capacitor, where the near-surface regionof a doped semiconductor substrate acts as one terminal of thecapacitor, and a gate layer acts as the other terminal. A gate oxidearranged between the gate layer and the semiconductor substrate acts asthe capacitor dielectric.

MOS capacitors are commonly used in many circuit applications, such ascharge pump, and RC-delay/trigger circuitry. However, the siliconsubstrate estate taken by the MOS capacitors may be significant, andaccordingly MOS capacitors may consume a large part of the total area ofan integrated circuit. Hence, decreasing the size of the capacitordevices or increasing the capacitance density of the capacitor devicesmay be desired to produce an integrated circuit in a smaller size.

SUMMARY

According to various non-limiting embodiments, there may be provided asemiconductor device. The semiconductor device may include a substrateincluding: a first semiconductor layer having a well region arrangedwithin the first semiconductor layer, a buried insulator layer arrangedover the first semiconductor layer, and a second semiconductor layerarranged over the buried insulator layer. The semiconductor device mayfurther include a capacitive structure, wherein the capacitive structuremay include the well region, at least one contact to the well region, atleast a portion of the buried insulator layer over the well region, atleast a portion of the second semiconductor layer, a source region and adrain region arranged over the second semiconductor layer, a gatedielectric layer arranged over the second semiconductor layer andarranged laterally between the source region and the drain region, and agate layer arranged over the gate dielectric layer. The well region, thesource region, and the drain region may have the same conductivity type.

According to various non-limiting embodiments, there may be provided amethod of forming a semiconductor device. The method may includeproviding a substrate, wherein the substrate includes a firstsemiconductor layer, a second semiconductor layer, and a buriedinsulator layer arranged between the first semiconductor layer and thesecond semiconductor layer. The method may further include forming awell region within the first semiconductor layer; forming at least onecontact to the well region; forming a source region and a drain regionover the second semiconductor layer; forming a gate dielectric layerover the second semiconductor layer and laterally between the sourceregion and the drain region; and forming a gate layer arranged over thegate dielectric layer, thereby forming a capacitive structure at leastpartially within the substrate. The well region, the source region andthe drain region may have the same conductivity type.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. Embodiments of theinvention will now be illustrated for the sake of example only withreference to the following drawings, in which:

FIG. 1 shows a schematic diagram illustrating a cross-sectional view ofa semiconductor device according to various non-limiting embodiments.

FIG. 2 shows a cross-sectional view of a semiconductor device accordingto various non-limiting embodiments.

FIG. 3 shows a perspective view of the semiconductor device of FIG. 2according to various non-limiting embodiments.

FIG. 4 shows a top view of the semiconductor device of FIG. 2 accordingto various non-limiting embodiments.

FIG. 5 shows an equivalent circuit of the semiconductor device of FIGS.2-5 according to various non-limiting embodiments.

FIG. 6 shows a top view of a semiconductor device according to variousnon-limiting embodiments.

FIG. 7 shows a flowchart illustrating a method of forming asemiconductor device according to various non-limiting embodiments.

FIGS. 8A-8G illustrate a method of forming a semiconductor deviceaccording to various non-limiting embodiments.

FIGS. 9A-9I illustrate a method of forming a semiconductor deviceaccording to various non-limiting embodiments.

FIG. 10 shows a cross-sectional view of a semiconductor device accordingto various non-limiting embodiments.

FIG. 11 shows a cross-sectional view of a semiconductor device accordingto various non-limiting embodiments.

DETAILED DESCRIPTION

Aspects of the present invention and certain features, advantages, anddetails thereof, are explained more fully below with reference to thenon-limiting examples illustrated in the accompanying drawings.Descriptions of well-known materials, fabrication tools, processingtechniques, etc., are omitted so as not to unnecessarily obscure theinvention in detail. It should be understood, however, that the detaileddescription and the specific examples, while indicating aspects of theinvention, are given by way of illustration only, and are not by way oflimitation. Various substitutions, modifications, additions, and/orarrangements, within the spirit and/or scope of the underlying inventiveconcepts will be apparent to those skilled in the art from thisdisclosure.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” is not limited to the precise valuespecified. In some instances, the approximating language may correspondto the precision of an instrument for measuring the value.

The terminology used herein is for the purpose of describing particularexamples only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise” (andany form of comprise, such as “comprises” and “comprising”), “have” (andany form of have, such as “has” and “having”), “include (and any form ofinclude, such as “includes” and “including”), and “contain” (and anyform of contain, such as “contains” and “containing”) are open-endedlinking verbs. As a result, a method or device that “comprises,” “has,”“includes” or “contains” one or more steps or elements possesses thoseone or more steps or elements, but is not limited to possessing onlythose one or more steps or elements. Likewise, a step of a method or anelement of a device that “comprises,” “has,” “includes” or “contains”one or more features possesses those one or more features, but is notlimited to possessing only those one or more features. Furthermore, adevice or structure that is configured in a certain way is configured inat least that way, but may also be configured in ways that are notlisted.

The term “connected” (or “coupled”) herein, when used to refer to twophysical elements, means a direct connection between the two physicalelements or a connection through one or more intermediary elements.

It should be understood that the terms “on”, “over”, “under”, “top”,“bottom”, “down”, “side”, “back”, “left”, “right”, “front”, “lateral”,“side”, “up”, “down” etc., when used in the following description areused for convenience and to aid understanding of relative positions ordirections, and not intended to limit the orientation of any device, orstructure or any part of any device or structure. In addition, thesingular terms “a”, “an”, and “the” include plural references unlesscontext clearly indicates otherwise. Similarly, the word “or” isintended to include “and” unless the context clearly indicatesotherwise.

The non-limiting embodiments described below in context of the devicesare analogously valid for the respective methods, and vice versa.Furthermore, it will be understood that the embodiments described belowmay be combined; for example, a part of one embodiment may be combinedwith a part of another embodiment.

It will be understood that any property described herein for a specificdevice may also hold for any device described herein. It will beunderstood that any property described herein for a specific method mayalso hold for any method described herein. Furthermore, it will beunderstood that for any device or method described herein, notnecessarily all the components or steps described must be enclosed inthe device or method, but only some (but not all) components or stepsmay be enclosed.

Various non-limiting embodiments relate to a semiconductor device, forexample, a capacitor device, such as a MOS (metal-oxide-semiconductor)capacitor (MOSCAP) device.

FIG. 1 shows a schematic diagram illustrating a cross-sectional view ofa semiconductor device 100 according to various non-limitingembodiments.

As shown in FIG. 1, the semiconductor device 100 may include a substrate110. The substrate 110 may include a first semiconductor layer 112, aburied insulator layer 114 arranged over the first semiconductor layer112, and a second semiconductor layer 116 arranged over the buriedinsulator layer 114. The first semiconductor layer 112 may include awell region 122 arranged within the first semiconductor layer 112. Thesemiconductor device 100 may further include a capacitive structure 120,wherein the capacitive structure 120 may include the well region 122, atleast one contact 124 to the well region 122, at least a portion of theburied insulator layer 114 over the well region 122, at least a portionof the second semiconductor layer 116, a source region 132 and a drainregion 134 arranged over the second semiconductor layer 116, a gatedielectric layer 126 arranged over the second semiconductor layer 116and arranged laterally between the source region 132 and the drainregion 134, and a gate layer 128 arranged over the gate dielectric layer126. The well region 122, the source region 132, and the drain region134 may have the same conductivity type, for example, one of N-type orP-type.

According to various non-limiting embodiments, the well region 122 maybe electrically coupled to the gate layer 128 through the at least onecontact 124 to the well region 122. The source region 132 may beelectrically coupled to the drain region 134.

According to various non-limiting embodiments, the semiconductor device100 may include an isolation element 140 configured to isolate the atleast one contact 124 from the source region 132 and the drain region134. The isolation element 140 may include a shallow trench isolation(STI) or a dielectric spacer. In various non-limiting embodiments shownin FIG. 1, the isolation element 140 may include a dielectric spacer,which may include a dielectric material, such as silicon nitride. Invarious non-limiting embodiments as shown in FIG. 11 below, theisolation element 140 may include a STI.

The semiconductor device 100 may include a further isolation element 142configured to isolate the gate layer 128 from the source region 132 andthe drain region 134. The further isolation element 142 may include adielectric spacer, such as a silicon nitride spacer, or may include aSTI arranged between the gate layer 128 and the source/drain region.

According to various non-limiting embodiments, the capacitive structure120 may include a first capacitor formed by at least the well region122, the buried insulator layer 114 (e.g. at least a portion of theburied insulator layer 114 arranged over the well region 122), and thesecond semiconductor layer 116 (e.g., at least a portion of the secondsemiconductor layer 116 arranged over the well region 122). The buriedinsulator layer 114 may be in direct contact with the well region 122.

The capacitive structure 120 may further include a second capacitorformed by at least the second semiconductor layer 116 (e.g. the portionof the second semiconductor layer 116 arranged over the well region122), the gate dielectric layer 126, and the gate layer 128.

According to various non-limiting embodiments, a dual gate field effecttransistor may be formed by at least the first capacitor, the secondcapacitor, the source region 132, and the drain region 134.

In various non-limiting embodiments, the well region 122 and the gatelayer 128 may be electrically coupled to a first terminal, and thesource region 132 and the drain region 134 may be electrically coupledto a second terminal. In other words, the well region 122 and the gatelayer 128 may be electrically connected to each other, and the sourceregion 132 and the drain region 134 may be electrically connected toeach other. In this connection, the first capacitor may be connected tothe second capacitor in parallel. The dual gate field effect transistormay be configured to provide a capacitance between the first terminaland the second terminal, e.g. the sum of the capacitances of the firstcapacitor and the second capacitor.

A primary gate may include at least the gate layer 128 configured togenerate a channel between the source region 132 and the drain region134 of the dual gate field effect transistor; and a secondary gate mayinclude at least the well region 122 configured to generate a channelbetween the source region 132 and the drain region 134 of the dual gatefield effect transistor.

According to various non-limiting embodiments, a thickness of the buriedinsulator layer 114 may be larger than a thickness of the gatedielectric layer 126. The buried insulator layer 114 may be a thininsulator layer having a thickness in a range from about 10 nm to about30 nm. In a non-limiting example, the buried insulator layer 114 mayhave a thickness of about 20 nm. The gate dielectric layer 126 may havea thickness in a range from about 2 nm to about 4 nm. In variousnon-limiting embodiments, the gate dielectric layer 126 may include ahigh-k dielectric material, such as hafnium dioxide (HfO₂), zirconiumdioxide (ZrO₂), titanium dioxide (TiO₂) or combinations thereof, whichhas “k” values higher than 3.9.

The second semiconductor layer 116 may be a thin active layer arrangedon the buried insulator layer 114. In various non-limiting embodiments,the second semiconductor layer 116 may have a thickness of about 12 nmor less, e.g. in a range from about 6 nm to about 12 nm.

According to various non-limiting embodiments, the semiconductor device100 may further include a salicide layer (not shown in FIG. 1) arrangedover the gate layer 128, the contact 124, the source region 132, and thedrain region 134. The salicide layer provided on the gate layer 128 mayhelp to reduce a high parasitic resistance incurred by un-salicided gatelayer 128, which in return reduces an equivalent series resistance (ESR)in the capacitive structure. In various non-limiting embodiments, thesalicide layer may include nickel-silicon (NiSi) salicide.

According to various non-limiting embodiments, the semiconductor device100 may further include a metal gate layer (not shown in FIG. 1)arranged between the gate layer 128 and the gate dielectric layer 126.The gate layer 128 may include polycrystalline silicon, and may have athickness of about 30 nm. In various non-limiting embodiments, the metalgate layer may include titanium nitride (TiN), and may have a thicknessof about 0.5 nm to about 1.5 nm. In various non-limiting embodiments,the metal gate layer may include aluminum (Al), and may have a thicknessof about 0.2 nm.

For more advanced technology 28 nm and below, the high-k insulator withmetal gate (HKMG) technology may be used to form the gate dielectriclayer 126 and the metal gate layer, which may increase the gatecapacitance, reduce leakage and increase the performance of the MOScapacitive structure formed therefrom. In addition, the MOS capacitivestructure may be formed in a part of MOSFET construction process,without requirement of additional process, mask and cost.

According to various non-limiting embodiments, the second semiconductorlayer 116 may be un-doped, or may have the same conductivity type withthe well region 122, the source region 132, and the drain region 134. Invarious non-limiting embodiments, the second semiconductor layer 116 maybe a thin silicon film of a fully-depleted silicon-on-insulator (FD-SOI)substrate 110 used to form a channel, which eliminates the need to havethe channel doped, making it fully depleted, i.e. a normally off device.The control of channel conduction may be carried out by the gatecontrol, allowing an ultra-low threshold voltage. In variousnon-limiting embodiments, the second semiconductor layer 116 may bedoped to have the same conductivity type as the well region 122, thesource region 132, and the drain region 134. In a non-limitingembodiment, the second semiconductor layer 116 may be doped with N-typedopants to form a NMOS capacitor structure with a N-type channel, whichmay be referred to as a MOS varactor.

In various non-limiting embodiments, the second semiconductor layer 116may be doped with N-type dopants, such as phosphorous. The well region122, the source region 132, and the drain region 134 may also be dopedwith N-type dopants.

According to various non-limiting embodiments, the well region 122 mayhave a doping concentration in a range from about 5e15 cm⁻³ to about5e16 cm⁻³. The source region 132 and the drain region 134 may have adoping concentration in a range from about 1e19 cm⁻³ to about 1e20 cm⁻³.The at least one contact 124 to the well region 122 may also be dopedwith N-type dopants with a doping concentration in a range from about1e19 cm⁻³ to about 1e20 cm⁻³. Accordingly, the capacitive structure 120may be a N-type capacitor (NCAP) structure. It is understood that theconductivity type of the well region 122, the source/drain region 132,134 and the contact 124 may also be P-type to form a P-type capacitor(PCAP) structure in other non-limiting embodiments, where the secondsemiconductor layer 116 may be un-doped or may be doped with P-typedopants to form the P-type capacitor structure with a P-type channel.

The first semiconductor layer 112 may be doped with a conductivity typedifferent from the conductivity type of the well region, the sourceregion and the drain region. The first semiconductor layer 112 may havea doping concentration of about 1e15 cm⁻³. In a non-limiting embodiment,the first semiconductor layer 112 may be a P-substrate layer doped withboron at the doping concentration of about 1e15 cm⁻³.

According to various non-limiting embodiments, the first semiconductorlayer 112 of the substrate 110 may include silicon (Si), germanium (Ge),silicon-germanium (SiGe), or combinations thereof.

The buried insulator layer 114 of the substrate 110 may include aninsulating material, such as silicon oxide (SiO₂), silicon nitride(SiN), or combinations thereof.

According to various non-limiting embodiments, the second semiconductorlayer 116 of the substrate 110 may include silicon, germanium,silicon-germanium, indium gallium arsenide (InGaAs), or combinationsthereof.

The second semiconductor layer 116 of the substrate 110 may include asingle-crystalline semiconductor material.

According to various non-limiting embodiments, the substrate 110 may bea semiconductor-on-insulator substrate. The semiconductor-on-insulatorsubstrate 110 may be provided or formed such that the secondsemiconductor layer 116 of the substrate 110 may be a single-crystallinesemiconductor layer. Examples of the semiconductor-on-insulatorsubstrate 110 may include but are not limited to a silicon-on-insulator(SOI) substrate, a germanium-on-insulator (GeOI) substrate, aSiGe-on-insulator (SiGe-OI) substrate, a InGaAs-on-insulator (InGaAs-OI)substrate, or combinations thereof.

In various non-limiting embodiments, the substrate 110 may include asilicon-on-insulator (SOI) substrate, wherein the second semiconductorlayer 116 may include single-crystalline silicon.

The substrate 110 may include a fully depleted silicon-on-insulator(FD-SOI) substrate, or a partially depleted silicon-on-insulatorsubstrate (PD-SOI) substrate. In various non-limiting embodiments wherethe capacitive structure may be formed along with MOSFET structures onthe FD-SOI substrate, the FD-SOI substrate may utilize back gate tocontrol the threshold voltage, i.e. turning on of the active MOSFETdevice, and accordingly require a thin buried insulator layer (e.g. witha thickness of about 10-30 nm).

According to various non-limiting embodiments, the substrate 110 mayinclude a SOI substrate whose surface is a face (100). For the MOScapacitor structure, there is no restriction or requirement on a crystalplane orientation of the substrate 110, so that an impurityconcentration, film thickness, dimension ratio of the device and thelike may be appropriately adjusted to obtain a capacitancecharacteristic that is suitable according to the plane orientation setby other requirements. In various non-limiting embodiments of a PMOSsemiconductor device (e.g. where the semiconductor device 100 furtherincludes a PMOS transistor), a surface of the PMOS semiconductor devicemay use a face (110), or a hybrid plane orientation substrate whosesurface is mixed by a face (100) and a face (110) may be used, in orderto improve performance of the PMOS semiconductor device.

According to various non-limiting embodiments, the second semiconductorlayer 116 may include a plurality of blocks arranged parallel to eachother along a first direction, and the gate layer may include aplurality of strips arranged parallel to each other along a seconddirection perpendicular to the first direction, as illustrated in FIG. 6below. Accordingly, the capacitive structure 120 may be an area typecapacitive structure, or a peripheral (finger) type capacitivestructure.

According to various non-limiting embodiments, the semiconductor device100 includes a capacitive structure including the first capacitor andthe second capacitor, which provide a higher density MOS capacitance andat the same time reduce the substrate area occupied by conventionallower density capacitive structure. The higher density capacitivestructure may be used in many applications, for example, in a voltagedoubler charge pump circuitry or a RC-delay/trigger circuitry.

FIG. 2 shows a cross-sectional view of a semiconductor device 200according to various non-limiting embodiments. FIG. 3 shows aperspective view of the semiconductor device 200 of FIG. 2 according tovarious non-limiting embodiments, and FIG. 4 shows a top view of thesemiconductor device 200 of FIG. 2 according to various non-limitingembodiments. The semiconductor device 200 is similar to thesemiconductor device 100 of FIG. 1, and thus the common features arelabeled with the same reference numerals. Various embodiments describedwith reference to FIG. 1 are analogously valid for the semiconductordevice 200 of FIGS. 2-4, and vice versa.

Similar to the semiconductor device 100, the semiconductor device 200may include the substrate 110 having the first semiconductor layer 112,the buried insulator layer 114 arranged over the first semiconductorlayer 112, and the second semiconductor layer 116 arranged over theburied insulator layer 114. The first semiconductor layer 112 mayinclude the well region 122 arranged within the first semiconductorlayer 112. The semiconductor device 100 may further include thecapacitive structure 120, wherein the capacitive structure 120 mayinclude the well region 122, the at least one contact 124 to the wellregion 122, at least a portion of the buried insulator layer 114 overthe well region 122, at least a portion of the second semiconductorlayer 116, the source region 132 and the drain region 134 arranged overthe second semiconductor layer 116, the gate dielectric layer 126arranged over the second semiconductor layer 116 and arranged laterallybetween the source region 132 and the drain region 134, and the gatelayer 128 arranged over the gate dielectric layer 126. The well region122, the source region 132, and the drain region 134 may have the sameconductivity type, for example, one of N-type or P-type.

As described in various non-limiting embodiments above, the substrate110 may be a semiconductor-on-insulator substrate, which may be providedor formed such that the second semiconductor layer 116 of the substrate110 may be a single-crystalline semiconductor layer. Examples of thesemiconductor-on-insulator substrate 110 may include but are not limitedto a silicon-on-insulator (SOI) substrate, a germanium-on-insulator(GeOI) substrate, a SiGe-on-insulator (SiGe-OI) substrate, aInGaAs-on-insulator (InGaAs-OI) substrate, or combinations thereof. In anon-limiting example, the substrate 110 may be a silicon-on-insulatorsubstrate, wherein the second semiconductor layer 114 may includesingle-crystalline silicon.

According to various non-limiting embodiments, the semiconductor device200 may further include a metal gate layer 236 arranged between the gatelayer 128 and the gate dielectric layer 126. In various non-limitingembodiments, the metal gate layer 236 may include titanium nitride, andmay have a thickness of about 0.5 nm to about 1.5 nm. In variousnon-limiting embodiments, the metal gate layer may include aluminum, andmay have a thickness of about 0.2 nm.

The high-k insulator with metal gate (HKMG) technology may be used toform the gate dielectric layer 126 and the metal gate layer 236 fortechnology of 28 nm and below, which may increase the gate capacitance,reduce leakage and increase the performance of the MOS capacitorstructure formed therefrom. In addition, the MOS capacitor structure maybe formed in a part of MOSFET construction process utilizing the high-kinsulator with metal gate process, without requirement of additionalprocess, mask and cost.

According to various non-limiting embodiments, the semiconductor 200 mayfurther include a salicide layer 238 arranged over the gate layer 128.The salicide layer 238 provided on the gate layer 128 may help to reducea high parasitic resistance incurred by un-salicided gate layer 128,which in return reduces an equivalent series resistance (ESR) in thecapacitive structure. The salicide layer 238 may include nickel-silicon(NiSi) salicide. The salicide layer 238 may also be arranged over thecontact 124, the source region 132 and the drain region 134 to providethe above-mentioned advantages, as shown in FIG. 2.

According to various non-limiting embodiments, the contact 124 may bearranged on a top surface of the well region 122, or may be at leastpartially arranged in the well region 122 as shown in the non-limitingembodiments of FIG. 10 below.

The well region 122 may be electrically coupled to the gate layer 128through the contact 124 to the well region 122. The source region 132may be electrically coupled to the drain region 134.

According to various non-limiting embodiments, the semiconductor device200 may include the isolation element 140 configured to isolate thecontact 124 from the source region 132 and the drain region 134. Theisolation element 140 may include a shallow trench isolation (STI) or adielectric spacer. In various non-limiting embodiments shown in FIGS.2-4, the isolation element 140 may include a dielectric spacer, whichmay include a dielectric material, such as silicon nitride. In variousnon-limiting embodiments as shown in FIG. 11 below, the isolationelement 140 may include a STI.

The semiconductor device 200 may include the further isolation element142 configured to isolate the gate layer 128 from the source region 132and the drain region 134. The further isolation element 142 may includea dielectric spacer, such as a silicon nitride spacer, or may include aSTI arranged between the gate layer 128 and the source/drain region.

According to various non-limiting embodiments, additional isolationelements 244 may be arranged at least partially around the periphery ofthe capacitive structure 120 of the semiconductor device 200 and atleast partially in the first semiconductor layer 112, so as to isolatethe capacitive structure 120 of the semiconductor device 200 fromadjacent components, such as a further capacitive structure having asimilar structure as the capacitive structure 120 show in FIG. 2 andarranged in the same substrate 110, and/or one or more MOSFETs arrangedin the same substrate 110 as shown in the non-limiting embodiments ofFIG. 10 and FIG. 11 below.

According to various non-limiting embodiments, the capacitive structure120 may include a first capacitor C1 formed by at least the well region122, the buried insulator layer 114 (e.g. at least a portion of theburied insulator layer 114 arranged over the well region 122), and thesecond semiconductor layer 116 (e.g., at least a portion of the secondsemiconductor layer 116 arranged over the well region 122). The buriedinsulator layer 114 may be in direct contact with the well region 122.

The capacitive structure 120 may further include a second capacitor C2formed by at least the second semiconductor layer 116 (e.g. the portionof the second semiconductor layer 116 arranged over the well region122), the gate dielectric layer 126, the metal gate layer 236, and thegate layer 128.

According to various non-limiting embodiments, a dual gate field effecttransistor may be formed by at least the first capacitor C1, the secondcapacitor C2, the source region 132, and the drain region 134.

A primary gate may include at least the gate layer 128 configured togenerate a channel between the source region 132 and the drain region134 of the dual gate field effect transistor; and a secondary gate mayinclude at least the well region 122 configured to generate a channelbetween the source region 132 and the drain region 134 of the dual gatefield effect transistor.

The semiconductor device 200 may include one or more primary gateterminal contacts 250, source/drain terminal contacts 252, and secondarygate terminal contacts 254, which are configured to provide electricalconnection to the respective primary/secondary gate and the source/drainregions through a plurality of interconnects 256, e.g., vias 256arranged in a dielectric layer (not shown in FIGS. 2-4) arranged overthe substrate 110 and the capacitive structure 120.

According to various non-limiting embodiments, the primary gate 128 maybe electrically coupled to the secondary gate 122 through the primarygate terminal contact 250, the secondary gate terminal contact 254, theat least one contact 124, and the associated interconnects 256. Thesource region 132 may be electrically coupled to the drain region 134through the source/drain terminal contacts 252 and the associatedinterconnects 256.

FIG. 5 shows an equivalent circuit 500 of the semiconductor device 200of FIGS. 2-5 according to various non-limiting embodiments.

As shown in FIG. 5, the first capacitor C1 may be formed by at least thewell region 122, at least a portion of the buried insulator layer 114arranged over the well region 122, and at least a portion of the secondsemiconductor layer 116 arranged over the well region 122. A firstcapacitor terminal of the first capacitor C1 is provided with thesecondary gate terminal contact 254, and a second capacitor terminal ofthe first capacitor C1 is provided with the source/drain terminalcontacts 252.

The second capacitor C2 may be formed by at least the gate layer 128,the metal gate layer 236, the gate dielectric layer 126, and the portionof the second semiconductor layer 116 arranged over the well region 122.A first capacitor terminal of the second capacitor C2 is provided withthe primary gate terminal contact 250, and a second capacitor terminalof the second capacitor C2 is provided with the source/drain terminalcontacts 252.

In various non-limiting embodiments as shown in FIG. 5, the well region122 and the gate layer 128 may be electrically coupled to a firstterminal T1 by electrically connecting the primary gate terminal contact250 to the secondary gate terminal contact 254. The source region 132and the drain region 134 may be electrically coupled to a secondterminal T2 by electrically connecting the source/drain terminalcontacts 252. In this connection, the first capacitor C1 may beconnected to the second capacitor C2 in parallel, such that the sum ofthe capacitances of the first capacitor C1 and the second capacitor C2is provided by the capacitive structure of various non-limitingembodiments between the first terminal T1 and the second terminal T2.

In an illustrative example, the thickness of the gate dielectric layer126 may be about 3.5 nm, and the thickness of the buried insulator layer114 may be about 20 nm. The capacitance BCAP of the first capacitor C1may be approximately determined according to:

BCAP≈(d _(Gox) /d _(Box))*NCAP≈0.18*NCAP

wherein d_(Gox) represents the thickness of the gate dielectric layer126, d_(Box) represents the thickness of the buried insulator layer 114,and NCAP represents the capacitance of the second capacitor C2.

Accordingly, the capacitive structure 120 of various non-limitingembodiments in the semiconductor device 200 provides a combinedcapacitance CAP=BCAP+NCAP=1.18*NCAP. Compared to the conventional MOScapacitor devices which only provides a capacitance NCAP, the capacitivestructure of various non-limiting embodiments utilizes the substrate 100to provide an additional capacitance of about 18% in the above example.

In a non-limiting embodiment where the first semiconductor layer 112 andthe second semiconductor layer 116 both include silicon, the capacitivestructure 120 of the semiconductor device 200 may be referred to as astacked SISIP (Si-Insulator-Si-Insulator-Poly) MOS capacitor on SOItechnology, which utilizes the bulk silicon, the buried insulator andthe silicon layer on the buried insulator to provide an additionalcoupling capacitance.

Accordingly, the capacitive structure of various non-limitingembodiments provides a higher capacitance by utilizing the substrate 110to provide an additional capacitor C1, without increasing the substratearea occupied by the capacitive structure. Hence, the semiconductordevice 200 provides a higher density capacitance.

FIG. 6 shows a top view of a semiconductor device 600 according tovarious non-limiting embodiments. The semiconductor device 600 issimilar to the semiconductor device 100, 200 of FIGS. 1-5, and thus thecommon features are labeled with the same reference numerals. Variousembodiments described with reference to FIGS. 1-5 are analogously validfor the semiconductor device 600 of FIG. 6, and vice versa.

Similar to the semiconductor device 100, 200, the semiconductor device600 may include the second semiconductor layer 116 and the gate layer128, as part of the capacitive structure.

Different from the semiconductor device 100, 200, the secondsemiconductor layer 116 of the semiconductor device 600 may include aplurality of blocks (616 a, 616 b, . . . ) arranged parallel to eachother along a first direction 602. The plurality of blocks may be spacedapart from each other. The gate layer 128 of the semiconductor device600 may include a plurality of strips (628 a, 628 b, 628 c, 628 d, 628e, . . . ) arranged parallel to each other along a second direction 604perpendicular to the first direction 602. The plurality of strips may bereferred to as fingers, and may be spaced apart from each other.Accordingly, the capacitive structure of the semiconductor device 600may be a peripheral (finger) type capacitive structure, with amultiple-finger in multiple-block configuration.

Although the non-limiting embodiments of FIG. 6 show the secondsemiconductor layer 116 having two blocks and the gate layer 128 havingfive strips, it is understood that any suitable number of blocks orstrips may be included in the semiconductor device 600 according tovarious non-limiting embodiments.

FIG. 7 shows a flowchart 700 illustrating a method of forming asemiconductor device according to various non-limiting embodiments. Themethod may be used to form the semiconductor device 100, 200, 600described in various non-limiting embodiments above. Variousnon-limiting embodiments described in context of the semiconductordevice 100, 200, 600 are analogously valid for the respective method,and vice versa.

At 702, a substrate may be provided. The substrate may include a firstsemiconductor layer, a second semiconductor layer, and a buriedinsulator layer arranged between the first semiconductor layer and thesecond semiconductor layer.

At 704, a well region may be formed within the first semiconductorlayer.

At 706, at least one contact to the well region may be formed.

At 708, a source region and a drain region may be formed over the secondsemiconductor layer.

At 710, a gate dielectric layer may be formed over the secondsemiconductor layer and laterally between the source region and thedrain region.

At 712, a gate layer may be formed over the gate dielectric layer,thereby forming a capacitive structure at least partially within thesubstrate. The well region, the source region and the drain region mayhave the same conductivity type.

It is understood that the method may not be carried out in the sequenceof 702-712 according to various embodiments. For example, the forming ofthe gate layer at 712 may be carried out before the forming of thesource region and the drain region at 708 according to a non-limitingembodiment.

As described in various non-limiting embodiments above, the substratemay be provided as a semiconductor-on-insulator substrate, which may beprepared or formed such that the second semiconductor layer of thesubstrate may be a single-crystalline semiconductor layer. For example,the semiconductor-on-insulator substrate may be prepared or formed usingSIMOX (separation by implantation of oxygen) method, wafer bonding, orseed methods. Examples of the semiconductor-on-insulator substrate mayinclude but are not limited to a silicon-on-insulator (SOI) substrate, agermanium-on-insulator (GeOI) substrate, a SiGe-on-insulator (SiGe-OI)substrate, a InGaAs-on-insulator (InGaAs-OI) substrate, or combinationsthereof. According to various non-limiting embodiments, the secondsemiconductor layer may include a single-crystalline semiconductormaterial. In a non-limiting example, the substrate may be asilicon-on-insulator substrate, wherein the second semiconductor layermay include single-crystalline silicon.

According to various non-limiting embodiments, the gate dielectric layermay be formed through a high-k insulator with metal gate process, or anoxidation furnace process.

According to various non-limiting embodiments, the source region and thedrain region may be formed through an epi-grown process, or animplantation process.

The method may further include forming a metal gate layer arrangedbetween the gate layer and the gate dielectric layer.

The method may further include forming a salicide layer over the gatelayer, the at least one contact to the well region, the source region,and the drain region.

Various non-limiting embodiments of the method of forming thesemiconductor device will be described in more detail below.

FIGS. 8A-8G illustrate a method of forming a semiconductor deviceaccording to various non-limiting embodiments. In the non-limitingembodiments of FIGS. 8A-8G, the method of forming the semiconductordevice 200 is illustrated. It is understood that the method of FIGS.8A-8G may be similarly used to form the semiconductor device 100, 600 ofFIG. 1 and FIG. 6. Various non-limiting embodiments described in contextof the semiconductor device above and the method of FIG. 7 areanalogously valid for the method of FIGS. 8A-8G, and vice versa.

As shown in FIG. 8A, the substrate 110 may be provided. The substrate110 may include the first semiconductor layer 112, the secondsemiconductor layer 116, and the buried insulator layer 114 arrangedbetween the first semiconductor layer 112 and the second semiconductorlayer 116.

In FIG. 8B, active regions 802, 804 of the substrate 110, referred to ashybrid regions, may be formed or defined using a mask. The activeregions 802, 804 may be the regions where one or more capacitivestructures (e.g. MOSCAP) of various non-limiting embodiments may beformed or located, or where one or more transistors (e.g. MOSFETs) maybe formed simultaneously with the capacitive structures. In variousnon-limiting embodiments, only one active region or more than two activeregions may be formed. In the description of FIGS. 8C-8G below, only oneactive region 802 is illustrated.

A first dielectric layer 860, such as a pad oxide layer of SiO₂ or a padnitride layer of SiN, may be formed over the second semiconductor layer116 for protection.

In FIG. 8C, the well region 122 may be formed, e.g. by ion implantation.The isolation elements 244 may also be formed for isolating the activeregion 802 from adjacent structures/devices. The isolation element 244may be formed as STI.

In FIG. 8D, the first dielectric layer 860 may be removed, and the gatedielectric layer 126 may be formed over the second semiconductor layer116. The gate dielectric layer 126 may include a high-k insulatingmaterial, such as hafnium dioxide (HfO₂), zirconium dioxide (ZrO₂),titanium dioxide (TiO₂) or combinations thereof, which has “k” valueshigher than 3.9.

In various non-limiting embodiments, the metal gate layer 236 may beformed over the gate dielectric layer 126. The metal gate layer 236 mayinclude TiN or Al. The gate layer 128, e.g., poly silicon gate layer,may be formed over the metal gate layer 236. The stack of the gatedielectric layer 126, the metal gate layer 236 and the gate layer 128may be formed and aligned with a width smaller than the width of theunderlying second semiconductor layer 116 through lithography process,so as to expose a portion of the second semiconductor layer 116 wherethe source and drain regions may be formed.

A second dielectric layer 862, e.g. the SiN spacer, may be formed overthe exposed surfaces of respective layers/regions, which may include thegate layer 128, a portion of the second semiconductor layer 116 and thewell region 122 as well as the isolation elements 244.

In FIG. 8E, the second dielectric layer 862 may be partially openedusing a mask, for subsequent formation of the contact 124 and thesource/drain regions 132, 134. At least one contact 124 may be formedover the well region 122, and the source region 132 and the drain region134 may be formed over the second semiconductor layer 116, through anepi-grown process, or an implantation process. The second dielectriclayer 862 may be remained between the contact 124 and the source/drainregion 132, 134 to form the isolation element 140.

In FIG. 8F, the second dielectric layer 862 may be partially opened overthe gate layer 128 for salicidation. The salicide layer 238 may beformed over the gate layer 128 by depositing nickel followed byannealing to form the NiSi layer 238. In various non-limitingembodiments, the salicide layer 238 may also be formed over the contact124, the source region 132 and the drain region 134. The seconddielectric layer 862 may be remained between the gate layer 128 and thesource/drain region 132, 134 to form the further isolation element 142.

In FIG. 8G, an interlayer dielectric 864, e.g. a layer of SiO₂, may beformed over the entire structure of FIG. 8F. Trenches may be opened inthe interlayer dielectric 864 and metal (e.g., tungsten) may be filledin the trenches to form the interconnects 256, for connection to thegate layer 128, the source/drain regions 132, 134, and the well region122. Further terminal contacts may be formed over the interconnects 256to form the semiconductor device 200 illustrated above.

FIGS. 9A-9I illustrate a method of forming a semiconductor deviceaccording to various non-limiting embodiments. In the non-limitingembodiments of FIGS. 9A-9I, the method of forming the semiconductordevice 200 is illustrated. It is understood that the method of FIGS.9A-9I may be similarly used to form the semiconductor device 100, 600 ofFIG. 1 and FIG. 6. Various non-limiting embodiments described in contextof the semiconductor device above and the methods of FIGS. 7 and 9A-9Iare analogously valid for the method of FIGS. 9A-9I, and vice versa.

In FIG. 9A, the method may be carried out using similar processes ofFIGS. 8A-8B to form the structure obtained in FIG. 8B, where a firstactive region 802 and a second active region 804 are formed.

In various non-limiting embodiments as described with reference to FIGS.9A-9I herein, a semiconductor device may be formed including thecapacitive structure of various non-limiting embodiments above formed inthe first active region 802, as well as MOSFETs formed in the secondactive region 804.

In FIG. 9B, a nitride hard mask 960 may be formed, for example, at leastsubstantially conformally formed, over the structure formed in FIG. 9A.A third dielectric layer 962 may be formed over the nitride hard mask960, and may be planarized. In the second active region 804, a firstsub-region 906 and a second sub-region 908 may be defined, wherein thefirst sub-region 906 may be used to form a NMOS transistor and thesecond sub-region 908 may be used to form a PMOS transistor, or viceversa. The second semiconductor layer 116 (e.g., a silicon layer) in thesecond sub-region 908 may be selectively processed, e.g. using a mask972, to form a layer of SiGe 970 in the second sub-region 908.

In FIG. 9C, the isolation elements 244, e.g., STI elements 244, may beformed at the peripheral of the first active region 802, to isolate thestructure in the first active region 802 from the structure in thesecond active region 804. Further isolation elements 944 may also beformed between the first sub-region 906 and the second sub-region 908 toisolate them from each other.

In FIG. 9D, the nitride hard mask 960 and the third dielectric layer 962may be removed. The well region 122 may be formed, e.g. by ionimplantation, within the first semiconductor layer 112 in the firstactive region 802.

In FIG. 9E, the first dielectric layer 860 may be removed, and the gatedielectric layer 126 may be formed over the second semiconductor layer116 and the SiGe layer 970. In various non-limiting embodiments, thegate dielectric layer 126 may include a high-k insulating material, suchas hafnium dioxide, zirconium dioxide, titanium dioxide, or combinationsthereof.

In various non-limiting embodiments, the metal gate layer 236, may beformed over the gate dielectric layer 126. The metal gate layer 236 mayalso be formed over the isolation elements 244, 944. The metal gatelayer 236 may include TiN or Al.

The gate layer 128, e.g., polysilicon gate layer, may be formed over themetal gate layer 236. A nitride layer 964 may be formed over the gatelayer 128, and an oxide layer 966 may be formed over the nitride layer964.

In FIG. 9F, the stack of the gate dielectric layer 126, the metal gatelayer 236 and the gate layer 128 in the first active region 802 may beformed and aligned with a width smaller than the width of the underlyingsecond semiconductor layer 116 through lithography process, so as toexpose a portion of the second semiconductor layer 116 where the sourceand drain regions for the capacitive structure may be formed.

Similarly, the stack of the gate dielectric layer 126, the metal gatelayer 236 and the gate layer 128 in the first sub-region 906 may beformed and aligned with a width smaller than the width of the underlyingsecond semiconductor layer 116 through lithography process, so as toexpose a portion of the second semiconductor layer 116 where the sourceand drain regions of the NMOS transistor may be formed. In the secondsub-region 908, the stack of the gate dielectric layer 126, the metalgate layer 236 and the gate layer 128 may be formed and aligned with awidth smaller than the width of the underlying SiGe layer 970 throughlithography process, so as to expose a portion of the SiGe layer 970.SiGe regions 974 may be formed on the exposed portion of the SiGe layer970 to form the source and drain regions of the PMOS transistor in thesecond sub-region 908.

A fourth dielectric layer 968, e.g. the SiN spacer, may be formed overthe exposed surfaces of respective layers/regions, which may include thegate layer 128, a portion of the second semiconductor layer 116 and thewell region 122 as well as the isolation elements 244.

In FIG. 9G, the fourth dielectric layer 968 may be partially openedusing a mask, for subsequent formation of the contact 124 and thesource/drain regions 132, 134. At least one contact 124 may be formedover the well region 122, and the source region 132 and the drain region134 may be formed over the second semiconductor layer 116 in the firstactive region 802, through an epi-grown process, or an implantationprocess. A source region 932 and a drain region 934 may also be formedover the second semiconductor layer 116 in the first sub-region 906. Thefourth dielectric layer 968 may be remained between the contact 124 andthe source/drain region 132, 134 to form the isolation element 140.

In FIG. 9H, the fourth dielectric layer 968 may be partially opened overthe gate layer 128 for salicidation. The salicide layer 238 may beformed over the gate layer 128 in the first region 802, the firstsub-region 906 and the second sub-region 908, by depositing nickelfollowed by annealing to form the NiSi layer 238. In variousnon-limiting embodiments, the salicide layer 238 may also be formed overthe contact 124, the source region 132, 932 and the drain regions 134,934, as well as the source and drain regions 974 of the PMOS transistor.The fourth dielectric layer 968 may be remained between the gate layer128 and the source/drain region 132, 932, 134, 934, 974 to form thefurther isolation element 142.

In FIG. 9I, the interlayer dielectric 864, e.g. a layer of SiO₂, may beformed over the entire structure of FIG. 9H. Trenches may be opened inthe interlayer dielectric 864 and metal (e.g., tungsten) may be filledin the trenches to form the interconnects 256, for connection to thegate layer 128, the source/drain regions, and the well region 122 in thefirst active region 802, the first sub-region 906 and the secondsub-region 908.

According to various non-limiting embodiments of FIGS. 9A-9I above, thesemiconductor device 900 is formed, including the capacitive structure120 of the semiconductor device 200 described above formed in the firstactive region 802, the NMOS transistor 910 formed in the firstsub-region 906, and the PMOS transistor 920 formed in the secondsub-region 908. It is understood that the semiconductor device 900including any number of capacitive structures and transistors may beformed in the same processes as described in various non-limitingembodiments above.

FIG. 10 shows a cross-sectional view of a semiconductor device 1000according to various non-limiting embodiments. The semiconductor device1000 is similar to the semiconductor device 200 of FIG. 2 and thesemiconductor device 900 of FIG. 9I. Various embodiments described withreference to FIG. 2 and FIGS. 9A-9I are analogously valid for thesemiconductor device 1000 of FIG. 10, and vice versa.

As shown in FIG. 10, the semiconductor device 1000 is similar to thesemiconductor device 900, except that the at least one contact 1024 tothe well region 122 in the capacitive structure 120 is formed within thewell region 122.

FIG. 11 shows a cross-sectional view of a semiconductor device 1100according to various non-limiting embodiments. The semiconductor device1100 is similar to the semiconductor device 200 of FIG. 2 and thesemiconductor device 900 of FIG. 9I. Various embodiments described withreference to FIG. 2 and FIGS. 9A-9I are analogously valid for thesemiconductor device 1100 of FIG. 11, and vice versa.

As shown in FIG. 11, the semiconductor device 1100 is similar to thesemiconductor device 900, except that the isolation elements 1140includes STI elements arranged at least partially within the substrateto isolate the contact 124 to the well region 122 from the source/drainregions 132, 134.

According to various non-limiting embodiments above, the semiconductordevice 100, 200, 600, 900, 1000, 1100 provides a higher capacitance byutilizing the substrate 110 to provide an additional capacitor C1,without increasing the substrate area occupied by the capacitivestructure. Hence, the semiconductor device of various non-limitingembodiments provides a higher density capacitance. The semiconductordevice 100, 200, 600, 900, 1000, 1100 of various non-limitingembodiments is advantageous in increasing the total capacitance of thecapacitive structure in Fully Depleted SOI process, and saving thesilicon substrate area. Further, no additional process step or mask isrequired to form the capacitive structure of various non-limitingembodiments, compared to the process of forming the MOS transistors.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments, therefore, are to be considered in all respectsillustrative rather than limiting the invention described herein. Scopeof the invention is thus indicated by the appended claims, rather thanby the foregoing description, and all changes that come within themeaning and range of equivalency of the claims are intended to beembraced therein.

1. A semiconductor device comprising: a substrate comprising: a firstsemiconductor layer, comprising a well region arranged within the firstsemiconductor layer; a buried insulator layer arranged over the firstsemiconductor layer; and a second semiconductor layer arranged over theburied insulator layer; and a capacitive structure, wherein thecapacitive structure comprises the well region, at least one contact tothe well region, at least a portion of the buried insulator layer overthe well region, at least a portion of the second semiconductor layer, asource region and a drain region arranged over the second semiconductorlayer, a gate dielectric layer arranged over the second semiconductorlayer and arranged laterally between the source region and the drainregion, and a gate layer arranged over the gate dielectric layer;wherein the well region, the source region, and the drain region havethe same conductivity type.
 2. The semiconductor device of claim 1,wherein the well region is electrically coupled to the gate layerthrough the at least one contact to the well region; wherein the sourceregion is electrically coupled to the drain region.
 3. The semiconductordevice of claim 1, further comprising an isolation element configured toisolate the at least one contact from the source region and the drainregion.
 4. The semiconductor device of claim 3, wherein the isolationelement comprises a shallow trench isolation or a dielectric spacer. 5.The semiconductor device of claim 1, wherein the capacitive structurecomprises a first capacitor formed by at least the well region, theburied insulator layer, and the second semiconductor layer, and whereinthe capacitive structure further comprises a second capacitor formed byat least the second semiconductor layer, the gate dielectric layer, andthe gate layer.
 6. The semiconductor device of claim 5, wherein a dualgate field effect transistor is formed by at least the first capacitor,the second capacitor, the source region, and the drain region.
 7. Thesemiconductor device of claim 6, wherein the well region and the gatelayer are electrically coupled to a first terminal, and wherein thesource region and the drain region are electrically coupled to a secondterminal, wherein the dual gate field effect transistor is configured toprovide a capacitance between the first terminal and the secondterminal.
 8. The semiconductor device of claim 6, wherein a primary gatecomprises at least the gate layer configured to generate a channelbetween the source region and the drain region of the dual gate fieldeffect transistor; wherein a secondary gate comprises at least the wellregion configured to generate a channel between the source region andthe drain region of the dual gate field effect transistor.
 9. Thesemiconductor device of claim 1, wherein a thickness of the buriedinsulator layer is larger than a thickness of the gate dielectric layer.10. The semiconductor device of claim 1, further comprising a salicidelayer arranged over the gate layer, the at least one contact, the sourceregion, and the drain region.
 11. The semiconductor device of claim 1,further comprising a metal gate layer arranged between the gate layerand the gate dielectric layer, wherein the gate layer comprisespolycrystalline silicon.
 12. The semiconductor device of claim 1,wherein the second semiconductor layer is un-doped or has the sameconductivity type with the well region, the source region, and the drainregion.
 13. The semiconductor device of claim 1, wherein the firstsemiconductor layer comprises silicon, germanium, silicon-germanium, orcombinations thereof.
 14. The semiconductor device of claim 1, whereinthe second semiconductor layer comprises silicon, germanium,silicon-germanium, indium gallium arsenide, or combinations thereof. 15.The semiconductor device of claim 1, wherein the buried insulator layercomprises silicon oxide, silicon nitride, or combinations thereof. 16.The semiconductor device of claim 1, wherein the substrate comprises afully depleted silicon-on-insulator substrate, or a partially depletedsilicon-on-insulator substrate.
 17. The semiconductor device of claim 1,wherein the second semiconductor layer comprises a plurality of blocksarranged parallel to each other along a first direction, wherein thegate layer comprises a plurality of strips arranged parallel to eachother along a second direction perpendicular to the first direction. 18.A method of forming a semiconductor device, the method comprising:providing a substrate, wherein the substrate comprises a firstsemiconductor layer, a second semiconductor layer, and a buriedinsulator layer arranged between the first semiconductor layer and thesecond semiconductor layer; forming a well region within the firstsemiconductor layer; forming at least one contact to the well region;forming a source region and a drain region over the second semiconductorlayer; forming a gate dielectric layer over the second semiconductorlayer and laterally between the source region and the drain region; andforming a gate layer over the gate dielectric layer, thereby forming acapacitive structure at least partially within the substrate; whereinthe well region, the source region and the drain region have the sameconductivity type.
 19. The method of claim 18, wherein the gatedielectric layer is formed through a high-k insulator with metal gateprocess, or an oxidation furnace process.
 20. The method of claim 18,wherein the source region and the drain region are formed through anepi-grown process, or an implantation process.